pacer: factor call stack searching into its own package

lib/caller/caller.go

@@ -0,0 +1,26 @@
+// Package caller contains functions to examine the call stack.
+package caller
+
+import (
+	"runtime"
+	"strings"
+)
+
+// Present looks for functionName in the call stack and return true if found
+//
+// Note that this ignores the caller.
+func Present(functionName string) bool {
+	var pcs [48]uintptr
+	n := runtime.Callers(3, pcs[:]) // skip runtime.Callers, Present and caller
+	frames := runtime.CallersFrames(pcs[:n])
+	for {
+		f, more := frames.Next()
+		if strings.HasSuffix(f.Function, functionName) {
+			return true
+		}
+		if !more {
+			break
+		}
+	}
+	return false
+}